Fast electromagnetic wave and undulating electron beam interaction structure



Aprll 14, 1964 R. M. PHILLIPS 3,129,356

FAST ELECTROMAGNETIC WAVE AND UNDULATING ELECTRON BEAM INTERACTIONSTRUCTURE Filed March 28, 1959 5 Sheets-Sheet 1 II.E l N P951427 PMAA/P!INVENTOR. BY%

Aprll 4, 1964 R. M. PHILLIPS 2 FAST ELECTROMAGNETIC WAVE AND UNDULATINGELECTRON V BEAM INTERACTION STRUCTURE Filed March 28, 1959 5Sheets-Sheet 2 i IE II IE- INVENTOR.

Apnl 14, 1964 R. M. PHILLIPS 3,129,356

FAST ELECTROMAGNETIC WAVE AND UNDULATING ELECTRON BEAM INTERACTIONSTRUCTURE Filed March 28, 1959 5 SheetS-Sheet 3 K E] E4 K; EwAP VMPMZZ/QJ INVENTOR.

Apnl 14, 1964 R. M. PHILLIPS 3,129,356

FAST ELECTROMAGNETIC WAVE AND UNDULATING ELECTRON BEAM INTERACTIONSTRUCTURE Filed March 28, 1959 5 Sheets-Sheet 4 iii R 11M I M W? i m H41 W e I Q U IMHII x; H n l k L,

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INVENTOR. BY Z- Apnl 14, 1964 R. M. PHILLIPS 3,129,356

FAST ELECTROMAGNETIC wAvE AND UNDULATING ELECTRON BEAM INTERACTIONSTRUCTURE Filed March 28, 1959 5 Sheets-Sheet 5 20552; A/a4 A06 *3INVENTOR.

United States Patent 3,129,356 FAST ELECTRQMAGNETIC WAVE AND UNDU-LATING ELECTRON BEAM INTERACTION STRUCTURE Robert M. Phillips, RedwoodCity, Calif., assignor to General Electric Company, a corporation of NewYork Filed May 28, 1959, Ser. No. 816,540 25 Claims. (Cl. 31539.3)

This invention relates to high frequency energy interchange devices ofthe type wherein an interchange of energy takes place between a streamof electrons and an electromagnetic wave in a waveguide. The inventionincludes devices wherein the electromagnetic wave abstracts energy fromthe electron stream as in amplifiers, oscillators or the like as well asdevices wherein the electromagnetic wave imparts energy to the electronsin the stream as in the case of an electron accelerator.

In particular, the invention relates to the class of beamtypeinteraction devices in which it is not necessary either to employ aslow-wave circuit or to hide the stream from the wave over a portion ofits travel. The interaction used in these devices will be referred to asa fast-wave interaction to distinguish it from the interaction used inconventional high frequency energy interchange devices.

In the conventional high frequency amplifier or oscillator of thetraveling-Wave variety, of which the travelingwave tube, backward-waveamplifier and backward-wave oscillator are examples, the unidirectionalenergy of the electron stream is converted to radio frequency energythrough a continuous and cumulative interaction between the beam and acomponent of the electric field of the radio frequency wave. Thecontinuous and cumulative interaction is accomplished by slowing thevelocity at which the wave progresses axially along the length of theguide (termed the phase velocity v -for the particular wave underconsideration in the particular guide) to approximately the velocity ofthe stream and providing a component of electric field in the directionof stream travel. The field component then causes the electrons in thestream to form in bunches. The bunches form in a favorable phase of thewave and are slowed by the electric field. A portion of the beam kineticenergy is thus imparted to the radio frequency wave. The slowing of thewave phase velocity is accomplished in a number of ways. The most oftenused device for slowing the Wave is the helix, which may be thought ofas one Wire of a two-wire transmission line. The distance traveled by awave on the conductor coiled into the form of a helix is greater thanthe distance traveled down the axis of the helix, the path followed bythe beam. Hence, although the wave velocity along the coiled conductorisapproximately the velocity of light, the axial component of thisvelocity can be much less than that of light. Another circuit which isused to slow the phase velocity of the wave is the loaded waveguide. Theintroduction into a smooth waveguide of periodic loading obstaclescauses the wave to be broken up into a number of components,

able interaction and bunching. Serpentine waveguides are examples ofcircuits of this type. The action of these devices may be shownmathematically to be equivalent "ice to synchronizing with a spatialharmonic in a loaded waveguide, hence are considered here to be in thesame class.

In the conventional high frequency amplifier or oscillator of thestanding-wave variety, of which the klystron is the prime example, theunidirectional energy of the electron stream is converted to radiofrequency energy through an interaction between a stream and an electricfield component .of a standing wave at two or 'more discrete gaps. Thestanding wave is excited in a resonant cavity which surrounds theinteraction gap, and consists of two traveling-waves of equal amplitudetraveling in opposite directions. The stream is not exposed to (ishidden from) the standing wave in'the region between cavities (driftregion). In a two-cavity klystron amplifier, it is the purpose of thefirst cavity to impart a velocity modulation to the stream. This isconverted to current modulation in the drift region. In other words,electron bunches are formed in the drift region. The bunches ofelectrons are decelerated by the standing wave in the second cavity,thus giving up a portion of their kinetic energy to the radio frequencyenergy in that cavity. The process of exposing the bunched beam to theelectric field in a discrete gap can be shown to be equivalent tosynchronizing the beam with that spatial harmonic of the standing wavewhich travels in the direction of and with the velocity of the electronstream. Working from this picture, the short gap klystron can be seen tobe a special case of the extended interaction klystron which consists oftwo or more resonant slow-wave circuits of any of the types previouslydiscussed, supporting standing rather than traveling Waves. The actionof this device can be thought of as a cross between the conventionaltraveling-wave tube and the discrete gap klystron. The action of thefirst resonant slow-wave section is .to bunch the beam in a continuousfashion much as in the traveling-wave tube. The action of the secondresonant section is to extract energy from the bunches. The action ofthe electron accelerator may be said to be the reverse of thetraveling-wave amplifier. The electron bunches are maintained in such aphase in the wave that the Wave imparts energy to the beam.

All of the conventionalde-vices discussed above have in common a circuitor waveguide for slowing the phase 'velocity of the electromagnetic waveand providing a component of electric field along the axisof theelectron stream. The structure of the wave propagating guide isrelatively complex and'difficult to fabricate. This is particularly truewhen the frequency band of interest becomes higher as for millimeter andsubmillimeter wavelengths where the slow-wave circuit becomesvanishingly small. Inhigh power tubes, the slowwave circuit is difficultto cool regardless of the frequency. Thus, for high power and millimeterwavelengths, fabrication and cooling of the slow-wave transmission linepresent particularly acute problems.

The class of device proposed eliminates the problems associated with theslow-wave transmission line by providing'a structure in whichinteraction takes place between an electron stream and a fast Wave, i.e'., having a phase velocity greater than the velocity of light. in

such devices, the electron stream rather than the circuit is madeperiodic. The class of devices includes those in which the periodicitynecessary to'the interaction'is in beam position or beam velocity (speedand/ or direction). One difliculty with prior fast-wave interactionschemes is that they do not exhibit the first order axial bunching ofthe beam which is characteristic of conventional devices and hence arenot capable of the efficiency exhibited by the conventional devices.Bunching in such devices,

of the type wherein the energy interchange takes place between anelectron stream and an electromagnetic wave which may be a fast wave andwherein the axial energy of the electron stream is available.

Another object of the present invention is to provide an energyinterchange device of the electron stream type wherein the interactionbetween the electromagnetic wave and the electron stream results in afirst order axial bunching of electrons in the electron stream.

Another difliculty encountered in known fast-wave devices is that ofmaintaining the electron stream in a condition for obtaining aninterchange of energy between the electron stream and the fast wave. Onedifiiculty is that of maintaining the periodicity in the beam requiredfor interaction. The other is in focusing the beam. Part of thedifficulty involved in maintaining periodicity and in focusing thestream results from the existence of a componet of radio frequencyelectric field perpendicular to the natural focused path of travel ofelectrons in the stream. This component of force tends to deform anddefocus the beam. Another cause of difficulty in maintaining beamperiodicity is that periodicity is intimately related to the strength ofthe focusing force, thus, requiring that the strength of the focusingforce be exact.

Accordingly, a further object of the present invention is to provide ahigh frequency energy interchange device of the type under considerationwherein individual electrons in the stream are not necessarily subjectedto components of force perpendicular to their focused path of travel.

Another object of the invention is to provide such a device wherein thefocusing forces for the electron stream are not critical.

In carrying out the present invention, a radio frequency propagatingstructure which may be a fast-wave waveguide provides an interactionregion and an electron stream is projected along the length of theregion in such a manner that electrons in the stream have a transversevelocity, the direction of which varies periodically along the guide. Aradio frequency electric field is introduced in the waveguide having aphase velocity related to the stream velocity and a mode such that atransverse component of the radio frequency electric field produces amodulation in the transverse velocity of electrons in the stream whichmodulation is converted into modulation in the axial velocity by anon-time varying focusing force such as a non-time varying periodicmagnetic field. Such a field converts changes in the transverse momentumof the electrons in the stream into changes in the axial momentum Whileleaving the stream energy essentially unchanged. The momentum conversionthus changes transverse velocity modulation in the stream into axialelectron bunching, that is, bunching of electrons along .the axiallength of the stream. The transverse component of electric field in theradio frequency wave then abstracts energy from the transverse momentumof the beam. Thus, the ultimate source of energy causing the radiofrequency wave to grow is the axial velocity of the stream. Stated inanother way, it may be said that the interaction mechanism depends uponan intermediate momentum conversion, that is, conversion of the axialmomentum of the beam into transverse momentum with the subsequentinterchange of energy between the transverse components of electricfield in the radio frequency wave and transverse momentum of electronsin the stream.

The novel features which are believed to be characterist-ic of thisinvention are set forth with particularity in the appended claims. Theinvention itself, however, both as to its organization and method ofoperation, together with further objects and advantages thereof, maybest be understood by reference to the following description taken inconnection with the accompanying drawings in which:

FIGURE 1 is a perspective view of a model utilized in describing andillustrating the high frequency energy interchange mechanism of thepresent invention;

FIGURE 2 is a side elevation of a portion of the device of FIGURE 1;

FIGURE 3 is a plan view of a portion of the device of FIGURE 1;

FIGURE 4 is an end view of a coaxial waveguide utilized in describingthe interaction mechanism as applied to a guide of such configuration;

FIGURE 5 is a partially broken away perspective view of the coaxialdevice of FIGURE 4 showing electron trajectories therein;

FIGURE 6 is a central vertical longitudinal section through the coaxialguide of FIGURES 4 and 5 additionally illustrating means for focusingthe electron stream in the guide;

FIGURE 7 is an end view of a circular waveguide utilized in describingthe interaction mechanism of the present invention as applied with aguide of this configuration;

FIGURE 8 is a perspective view of the circular guide of FIGURE 7illustrating electron trajectories necessary for the interaction;

FIGURE 9 is a central vertical longitudinal section through a portion ofthe circular guide of FIGURE 8 illustrating the electron stream focusingarrangement;

FIGURE 10 is a partially broken away side elevation of a preferredembodiment of a high frequency energy interchange device utilizing thepresent invention;

FIGURE 11 is a plan View showing a portion of the waveguide employed inthe device of FIGURE 10 and its focusing system as seen along sectionlines 11--11;

FIGURE 12 is a transverse section through the rectangular body portionof the device of FIGURE 10 taken along section lines .1212 of FIGURE 11;

FIGURE 13 is a side elevation partially broken away and partially insection of a high frequency energy interchange structure which employsthe principles of the invention to obtain a fast wave interaction in anextended interact-ion klystron; and

FIGURE 14 is a transverse section through a coupling section taken alongline 14-14 of FIGURE 13.

In order to obtain an understanding of the principle of operation of thepresent invention, reference should be made to FIGURES l, 2 and 3 of thedrawings which schematically illustrate a waveguide 16 consisting of apair of parallel conductive planes 11 and 12 which are considered to beinfinite in extent for purposes of discussion. Let us assume that theguide 10 is excited by a radio frequency electromagnetic Wave of thetransverse electric mode known as TE This mode may be considered thefundamental mode of an ordinary rectangular waveguide which has itsnarrow dimension extended to infinity. By definition, the TE mode haslines of electric force or electric flux which are all parallel to thetwo parallel plates 11 and 12 and perpendicular to the axial dimensionof the guide 10 as indicated by the arrows E, and the lines of magneticflux are perpendicular to the lines of electric flux as indicated by thearrow H. Further, the electromagnetic wave is propagated down the lengthof the guide perpendicular to the lines of electric force E. There cannot be any tangential electric field at the surface of the planes i1 and12 because they are conducting planes. Therefore, the electric fieldintensity, 211- though everywhere horizontal in direction, diminishes tozero at the parallel surfaces 11 of the guide It). That is to say, thatthe electric field intensity is zero at the surface of the conductingplanes 11 and 12 and increases to a maximum at the center as indicatedby the flux density diagram marked E in FIGURE 1.

A sheet electron stream 13 which is to interact with the electromagneticWave propagated down the Waveguide is directed down the length of theregion between the two conducting plates 11 and 12. The phase velocity(v of the electromagnetic wave propagated down the guide is greater thanthe velocity of light (0) whereas electrons in the stream 13 must beless than the velocity of light. Utilizing the structure described thusfar, there is no net interaction between the electromagnetic wave andthe electron stream 13. The interaction mechanism utilized makes use ofan interchange of energy between electrons in the stream and transversecomponents of the radio frequency electric field E in theelectromagnetic wave rather than, as is the usual case, with the axialcomponents of the electromagnetic wave. This mechanism may best beunderstood by a consideration of the plan view of the guide 1% as seenin FIGURE 3. In considering FIGURE 3, it should be noted that therepresentation of the radio frequency electric field is an instantaneousone and therefore the electromagnetic wave appears to be stopped in itstravel along the length of the guidelt), whereas, in practice, theregions of electric field having vectors in a given direction move downthe length of the guide at the velocity which we have called the phasevelocity. Electrons in the sheet stream 13 are made to move from side toside in the waveguide 19 as they move down the axial length in order toobtain the desired interchange of energy. The transverseundulatingmotion of four such individual electrons 14, 15, 15 and 17 isillustrated in FIGURE 3 to facilitate description of the principle.

In order to cause the electrons in the stream 13 to have the transverseundulating motion illustrated, they may be subjected to a series ofnon-time varying spatially periodic magnetic focusing fields which havelines of force perpendicular to the conducting planes 11 and 12 of thewaveguide Iltl. Thus, the magnetic focusing fields of interest areperpendicular to the direction of propagation of the electromagneticwave, the direction of flow of the electron stream 13 and alsoperpendicular to the plane in which it is desired to cause electrons toundulate. As illustrated, the periodic magnetic focusing field isprovided by a series of magnets 18 which have poles located on oppositesides of the guide 19 adjacent the conducting planes in such a mannerthat a nort magnetic pole on one side of the guide is directly oppositea south magnetic pole on the opposite side of the guide It) andprogressing down the guide in the direction of propagation of theelectromagnetic wave, the polarity of themagnetic poles alternate. Thatis, for example, progressing down the top plane 11 of the guide 10 inthe direction of propagation first a north pole is encountered, then asouth pole, next, another north pole and then another south pole, and soon. Thus, the magnetic field always has a component perpendicular to theconducting planes 11 and 12. but alternates in the directional sense asindicated by the lines marked B in FIGURE 2 of the drawings.

Due to the fact that the force exerted by a magnetic field on a chargedparticle is always perpendicular to the direction of motion of theparticle, the electrons are not displaced toward or away from theparallel plates 11 and 12 of the guide as they move axially down theguide 19, but have a side-to side serpentine motion in the plane ofentry.

For example, as seen in FIGURE 3, the charged particles 14, 15, 16 and17 move into a magnetic field which is said to be down, i.e., from thetop to the bottom of the guide 10. Thus, the charged particlesstartto'move ina curved or circular path in the'clockwise direction.However,'before they curve all the way around, they move into a magneticfield which is in the opposite sense and are therefore caused to move ina curved path with the opposite rotation; that is, they move in a pathwhich rotates in a counterclockwise sense (the terms clockwise andcounterclockwise are used here to describe rotation as seen looking downfrom the top of the guide 10 as illustrated in FIGURE 3). Thus, theindividual charged particles move down the length of the guideundulating from side to side in the direction of the transverse electricfield lines E while maintaining their original plane of entry.

As has already been pointed out, a fixed relationship (a synchronism)must be maintained between the electrons in the electron stream and theinteraction component of the electromagnetic wave as they move down thewaveguide in order for a net interchange of energy, either from thestream to the wave, or vice versa, to take place.

Since the interaction in question involves electromagnet ic waves whichhave a phase velocity (v greater than the velocity of light (0) and anelectron stream in which electrons have a velocity less than that oflight, the condition responsibe for interaction deserves some specialcomment. In order to assist in the discussion, a coordinate system hasbeen superimposed on FIGURES 1, 2 and 3. As may be seen in FIGURE 1, theplanar surfaces 11 and 12 are parallel to the YZ plane with the electronstream 13 directed in the Z direction and the Y axis directed out of thepaper and parallel to the electric field lines E. The X axis then isperpendicular to the conductive planes 11 and 12. Thus, in the sideelevation of FIGURE 2, the paper represents the XZ plane while in theplan view of FIGURE 3, the paper represents the YZ plane. The termsperiod and periodic length are designated by the symbol :P and used todenote the interval or distance in the axial (2) direction betweencorresponding points on any closest two like pole pieces. For example,in FIGURE 2, P is designated as the distance in the Z or axial directionbetween center points of two north poles on the upper Wall 11 of theguide It) which are closest together.

In the fast-Wave interaction of the apparatus of FIG- URES 1, 2 and 3,the :condition which is called synchronous is obtained between theelectron stream and the electromagnetic wave by forcing the velocitydirection of the stream 13 to vary periodically at such a rate that anindividual electron in the stream travels one periodic length P in thetime that the wave travels one periodic length P plus one Wavelength ofthe electromagnetic wave.

This may best be understood :by consideration of the electrontrajectories illustrated in FIGURE 3. Since the :individual electronsundulate from side to side as they move down the waveguide, theirvelocity in the Y direction .(side-to-side velocity) must 'be zero atpoints where the electrons are changing directions. Half way betweenthese points, the velocity in the Y direction is maximum. If theelectrons in tthe stream are so phased that their velocityin" the Ydirection'is a maximum when the decelerating force due to the radiofrequency electric-field E is a maximum,.and, if :the electrons reversedirection at the same instantthat the radio frequencyelectric field Eseen by the electrons reverses direction, the electron will continue toexperience a periodicdecelerating force in the Y direction throughoutitstravelas long as the proper phase relationship is maintained. Theelectric field will-reverse each-time the electrons velocity reverses iftheelectron travels oneperiod (the distance P) in the time that theradio frequency wave travels one periodP plus one radio frequency wavelength.

Under the conditions described, the forces exeprienced by the electrons'14, 15, 16 and 17in the electron stream are such as to cause themtoimpart energy'to the electromagnetic wave. The electrons :14, 15, 16and17, considered, are selected in the most favorable phase. Aconsiderationof the principles described revealsthat, for thesynchronous condition described, other electrons, i.e., electrons in themost unfavorable phase, receive energy from the wave in similar fashionand electrons intermediate to two extreme phases impart or receiveenergy in varying degrees. Like the conventional traveling-wave tube,

the net energy interchange between the electron stream andelectromagnetic wave is zero for exact synchronism. Also, as in the caseof the conventional traveling-wave tube, there is a net transfer ofenergy from the stream to the wave when the electron stream has avelocity Which is slightly greater than the exact synchronous velocity.Thus, the condition for providing amplification is available.

An extremely important property of the interaction described is that theelectrons in the stream are formed into bunches along the axial lengthof the stream much as the bunches of electrons which occur inconventional traveling-wave tubes. It is felt that the presentdiscussion is not the place for a mathematical proof of this effect.However, in simple terms, it may be stated that this bunching ofelectrons is caused by the periodic magnetic field through which theelectrons must pass. The changes in the transverse beam momentum causedby the transverse electric fields are converted into changes in axialmomentum by the periodic magnetic field. In a sense, the magnetic fieldborrows energy from the axial velocity and turns it over to the radiofrequency electric field in the form of transverse velocity. Hence, theenergy available to the radio frequency wave is not simply therelatively small energy in the transverse motion but the much largeramount of energy introduced to accelerate the electrons in the axialdirection.

In order to verify these results, the equations of motion for anelectron in the undulating beam device illustrated were solved on ananalogue computer. Solutions were obtained for eight electrons equallyspaced in the radio frequency electromagnetic wave. The results of thestudy showed good first order axial bunching of the electrons in thestream and good efiiciency of conversion of beam energy to radiofrequency energy. Further, one of the preferred embodiments of the highfrequency energy interchange device (described subsequently)experimentally verifies the results obtained analytically.

The apparatus of FIGURES 1, 2 and 3 utilized thus far in describing theinteraction utilizes a waveguide defined between two conducting plates11 and 12 which are considered to be of infinite extent. Obviously, sucha device can not be built. However, the electric field configuration inthe guide as illustrated and the undulating electron stream can bereproduced in various ways. In other words, the relationship betweenelectron trajectories and electric fields described may be obtainedusing other configurations. The model illustrated in FIGURES 4, 5 and 6represents one such configuration. The model illustrated in FIGURES 4, 5and 6 represents one such configuration. The device of FIGURES 4, 5 and6 may be considered as a development of the device of FIGURES l, 2 and 3which is obtained by wrapping the planes 11 and 12 around an axiscentrally located beneath the lower conducting plane 12 and extendingparallel thereto in the axial (Z) direction. This development produces awaveguide 20 which consists of concentric conductive right circularcylinders or pipes 21 and 22. The Waveguiding portion comprises the areabetween the two concentric waveguiding pipes 21 and 22.

If the device of FIGURES 4, 5 and 6 is considered as a wrapped updevelopment of the device of FIGURES l, 2 and 3, it is seen that therequired radio frequency electric field lines must extendcircumferentially around the interior of the guide 20 and the electricfield must be of maximum intensity approximately midway between theindividual cylindrical conductors 21 and 22 as illustrated by the fieldlines E, and further, the electron stream must be a hollow cylindricalsheet stream in which individual electrons must be made to undulate fromside to side.

The electromagnetic wave necessary to produce the electric field whichhas circumferential components as illustrated by the lines marked E inFIGURE 4 is produced by a coaxial waveguide mode of the TE type. Aconventional hollow stream electron gun may be used to produce thestream of electrons within the waveguide, and periodic magnetic fieldswhich are radial may be used to cause electrons in the stream toundulate as the stream passes down the length of the waveguide.

The periodic magnetic field of the required configuration may befurnished in a number of Ways. One way, for example, is to providespaced apart disc-shaped radially magnetized magnets 23 inside theinside of the inner conductive pipe 22, and also provide disc-shapedmagnets 24 surrounding outer conductor 21 which are coaxial with respectto the inner magnetic discs 23, occupy the same plane, and are ofopposite polarity. Both the inner and the outer magnets 24 alternate inpolarity down the length of the guide 20 as illustrated in FIGURE 6.Thus, the periodic magnetic field is provided down the length of thewaveguide. The particular hollow stream electron gun utilized is notillustrated since any conventional hollow stream gun which does notcause the stream to spin may be used. For example, a hollow stream gunwhich might be used is illustrated and described in a paper entitled AC-W UI-IF 'IVVT Power Amplifier of Extended Bandwidth, by Ward A.Harman, published in pages 36 through 40 of the 1957 Proceedings of theNational Conference on Aeronautical Electronics, sponsored by theInstitute of Radio Engineers on May 13, 14 and 15, 1957, in Dayton,Ohio. It should be noted that the periodicity of the magnetic field inthe guide should bear the same relationship to the electron streamvelocity and electromagnetic wave as described in connection with theapparatus of FIG- URES 1, 2 and 3 to obtain the fast-Wave type ofinteraction described. For the coxial type waveguide 20 illustrated, ithas been found that an amplitude of undulations of electrons in thestream which is satisfactory to produce good interaction may be on theorder of one fourth of the radial distance between the inner and outerconductors 21 and 22 of the waveguide.

The configuration of the guide 20 has the advantage that the peakelectric interacting field as illustrated in FIGURE 4 for this device isremoved as far from each of the walls of the waveguide as is possible,i.e., a maximum distance away. Therefore, the electron stream may alsobe a maximum distance away from the walls of the guide. Further, thearrangement allows for easy adjustment of the periodicity of the magnetsto improve efficiency since the magnets are entirely external to thewaveguide which must ultimately be evacuated. These advantages arecommon to all of the structures which employ the present invention.

Another structure which may be used to obtain the correct fieldconfigurations for the interaction described, is illustrated in FIGURES7, 8 and 9 of the drawings. All of the elements of the structureillustrated in these figures are common to the device illustrated inFIGURES 4, 5 and 6; therefore, common components of the two devices aregiven like reference numerals in order to simplify the description anddrawings. The principal structural difference between the two devices isthat the device of FIGURES 7, 8 and 9 does not have the centralconductor 22 and its associated magnetic discs 23 as found in thepreviously described structure. In other Words, the main waveguide 20 isa simple hollow conductor of circular cross section. The magnetic fieldrequired is furnished by the external disc-shaped magnets 24. That is,the magnetic field thus produced will cause electrons in an electronstream directed down the waveguide to undulate as they pass down theguide rotating alternately in a clockwise and anticlockwise direction.

The interaction may be obtained using any one of the family of TEcircular waveguide modes although the most advantageous mode is the TEmode. The electric field for the TE mode is illustrated in the end viewof the circular guide of FIGURE 7. From an inspection of the electrontrajectories and the configuration of the electric field E in the guide20, it is seen that the condition for interaction described with respectto the planar guides of FIGURES 1, 2 and 3 may exist. That is to saythat, if the velocity direction of the electron stream is made to varyperiodically at such a rate that an individual electron in the streamtravels one periodic length P in the time that the electromagnetic Wavetravels one periodic length P plus one wavelength of the electromagneticwavelength, the synchronism required for interchange of energy betweenthe stream and wave exists. The structure has the advantages attributedto the two previously described fast-wave devices.

A preferred embodiment of the invention (the one previously referred to)is illustrated by FIGURES 10, 11 and 12 of the drawings. This device maybe considered to be a vertical longitudinal segment of the apparatusdescribed and illustrated in FIGURES 1, 2 and 3. Thus, the centralwaveguide portion 30 of the device constitutes a waveguide ofrectangular cross section.

In addition to the central waveguide portion 39, the traveling-wave tubeincludes an electron gun 33, which is encapsulated in one end for thepurpose of producing and directing a stream of electrons along the axisof the waveguide Si! and a cooled collector 134 located in an enlargedenclosure 4 at the opposite end of the tube for the purpose ofcollecting electrons from the gun 33.

In order to launch the electromagnetic wave in the waveguide portion35), an input waveguide section 27 of rectangular cross section ispositioned at the gun end of the guide 30 with its longitudinal axisperpendicular to the longitudinal axis of the main guide 3%. A window(not shown) is positioned within the input guide section 27 in order toprovide a vacuum tight seal. A similar guide 23 is positioned in a likemanner at the collector end of the device to provide an outputtransmission path for the amplified electromagnetic waves. Theinteracting electromagnetic wave launched within the guide 36 is thefundamental transverse electric mode for the "guide, i.e., the TE mode.

The gun 33 consists of a cathode 3S and a cathode heater 36 wmch isconnected to a suitable energizing source (not shown) and which causesthe cathode to emit electrons when heated. A centrally aperturedelectron stream focusing electrode 37 and a correspondingly centrallyapertured electron beam accelerating anode 38 are provided for causingthe electrons emitted by the cathode 35 to be projected outwardly alongthe axis of the waveguide structure 33 in a stream as depicted by brokenlines 39. In order to simplify the description and drawings, theenergizing voltage supply for the electron gun electrode is not shown.The electron stream produced by the gun 33 is of circular cross sectionbut electrons in the stream may be made to undulate as described inconnection with the discussion of the apparatus of FIGURES l, 2 and 3. I

As described in connection with the planar version of FIGURES l, 2 and3, the desired periodic magnetic field is provided by a series ofmagnets 31 located on opposite sides of the guide. The magnets 31 arepositioned ad jacent the narrow walls in such a manner that a magneticnorth pole on one side of the guide is directly opposite a magneticsouth pole on the opposite side of the guide and progressing down theguide in the direction of propagation of the electromagnetic wave, thepolarity of the magnetic poles alternate. For example, progressing downone narrow wall of the guide 30 in the direction of propagation ofelectromagnetic waves, first a nort magnetic pole is encountered, then asouth.p.ole, nex another north pole and then another south pole, and soon. In this manner, the magnetic field always has a componentperpendicular to the narrow walls of the guide 31) but alternates in thedirectional sense.

Thus, the configuration of radio frequency electric fields and electrontrajectories in the guide are similar to those described in connectionwith FIGURES l, 2 and 3. Consequently, the same interaction is obtained.A modiv 10 fication of the device of FIGURE 10 which can be made with aresultant reduction instream velocity is to bring the focusing magnets31 in closer to the axis of the waveguide 30. This provides a higherconcentration magnetic field between opposing pole pieces whichincreases the focusing force and allows the magnetic pitch P to bereduced.

In each of the devices described, a fast-wave interaction is providedwhich eliminates the need for a slow-wave circuit. Further, there anextremely large interaction area in each of the devices when comparedwith a conventional high frequency energy interchange device. The use ofsuch a guide substantially eliminates matchingand reflecting problemsbetween the main waveguide and input and output sections. The fact thatthe electrons in the streams undulate along the lines of electric fiuxofthe transverse electric field rather than across them eliminates a wholeclass of stream focusing problems commonly encountered in other knowndevices of the fast wave interaction type. Each of the structures alsoallows external variation of the position of focusing magnets whichmeans that the periodicity of undulations in the internal electronstream may be adjustedto improve efliciency. An additional advantage ofthe structureis that the peak intel-acting electric field is far fromthe wall. This minimizes stream interception problems and allows one toplace the beam at the point of maximum field. H

One structure in which the fast-wave interaction described is used toparticular advantage is the extended interaction klystron 4d illustratedin FIGURES 13 and 14. As previously indicated, the conventional klystronis dependent upon the exposureo-f an electron stream to the standingwave excited a resonant cavity to convert the unidirectional energy ofthe electron stream to radio frequency energy. The exposure of thestream to the wave must be accomplished in a narrow discrete. gap. Also,'as was previously pointed out, the action of the klystron is improvedin many respects by providing resonant slow-wave circuits, -i.e.,slow-wave circuits in resonant cavities, so that the electron streammaybe exposed to the electric field of the resonant cavity throughoutits travel in the cavity rather than being exposed to electric fields invery narrow discrete regions. Since the interaction mechanism of thepresent invention allows interaction between an electron stream and.fast electromagnetic waves, the interaction maybe obtained as describedwith respect'toFIGURES 1, 2 and 3 between an electron stream havingundulating electrons therein and electric fields in a cavity which doesnot have narrow discrete gaps and which does not contain a slow-wavecircuit.

The structure may best be seen by reference to FIG- URES l3 and 14-wherein the extended interaction klystron is illustrated without anelectron gun or a collector. The'gun and collector are broken away dueto the fact that they are exactly the same as those componentsillustrated inFIGURES 10, 11 and 12. Corresponding reference numeralsare used in FIGURES 10 and "13 to identify the position of theseelements.

Thus, the tube includes an electron gun not 'shown) which isencapsulated in one end for the purpose of producing and directing astream of electrons along the axis of the structure 40. A cooledcollector (not shown) is located in an enlarged enclosure at theopposite end of the tube for the purpose of collecting electrons fromthe gun. The interaction circuitry is located between the electron gunand the collector. The interacting circuitry includes an input cavity 41which has the configuration of a right circular cylinder with an openingin its opposite end walls to provide a passage for an electron stream42. The interaction circuitry also includes a hollow cylindricalpipe-like conductive portion 43 and an output cavity 44.

The input cavity may be considered a first resonant Waveguide much asthe cylindrical waveguide illustrated and described in connection withFIGURES 7, 8 and 9.

Like the cylindrical Waveguide described in connection with thosefigures, one of the most advantageous modes for interaction is the TEcircular waveguide mode. This is accomplished by the waveguide couplingsection 45 Which will be described in more detail subsequently. Sincethe electric field associated with the TE circular waveguide mode hasbeen previously described, the description will not be reiterated atthis point. However, it should be noted that the input waveguide cavity4-1 is made an integral number of half Wavelengths long in order toprovide the resonant condition desired. For example, the cavity 41illustrated is three halves wavelength long. Similarly the output cavity44 may be considered a second resonant Wave guide which may besubstantially identical to the above-described input cavity or firstreso nant waveguide 41.

In order to give electrons in the stream 42 the proper trajectories forinteraction, discs of radially magnetized material 46 are positionedaround the input cavity in such a manner that they produce the spatiallyperiodic magnetic fields necessary to cause electrons in the stream toundulate as described with respect to the circular device of FIGURES 7,8 and 9. For example, the first disc 46, do, the disc which produces themagnetic field first encountered by the electron stream 42, isillustrated as producing a magnetic north pole, the second disc, amagnetic south pole, and the third disc, a magnetic north pole. Onceagain, the velocity of the electron stream is selected so that electronsin the stream travel one period P as previously defined while theforward traveling component of the standing electromagnetic wave travelsone period plus one radio frequency wavelength. Thus the conditionspreviously described for interaction are met and the standing wave inthe input cavity pro duces a velocity modulation on electrons in theelectron stream.

The hollow conductive portion 4-3 which is located intermediate theinput and output cavities 41 and 44, respectively, constitutes anelectron drift channel where the electron stream is not exposed tostanding waves in either cavity. Consequently, the velocity modulationimparted to the electron stream is converted to current modulation inthis guide. The length of the channel 43 is selected to provide maximumconversion from velocity modulation of the stream to current modulation.For the apparatus illustrated, this distance is on the order of one toten times the cavity length. The current modulated electron stream 42then passes into the output cavity 44.

As previously pointed out, the output cavity or second resonantwaveguide 44 may be of substantially identical structure to the inputcavity or first resonant Waveguide 41. As a consequence, the currentmodulated electron stream excites the output cavity 44 and gives up aportion of its kinetic energy to the radio frequency energy in theoutput cavity 44. Although it is possible by utilizing specialtechniques to extract energy from the output cavity without causing theelectrons from the stream to experience several undulations within thatcavity, the best energy conversion is obtained by causing the electronsto undulate and thereby converting axialmomentum of the electron streaminto transverse momentum with the subsequent interchange of energybetween the transverse components of the electric field and transversemomentum I of electrons in the electron stream. Therefore, the output toa circular guide and vice versa. For the best understanding of theconfiguration of the input and output couplers 45 and 48, respectively,reference should be had to FIGURE 14. The coupler is made up of astraight section of rectangular waveguide 5t; which is brought intoanother waveguide or rectangular cross section Sll which has the form ofa torus. The straight section 5t) contains a vacuum tight window 56which provides a seal. The narrow Walls of the wrapped up portion iii ofthe coupler 45 form inner and outer Walls 53 and 54, respectively, whichdefine the inner and outer diameter of the torus 51. The inner Walldefines a centrally located aperture through the coupling structure toaccommodate an electron stream. The one flat broad wall of the circularportion 51 of the input coupler 45 which is adjacent the input cavity4-1 is provided with four coupling slots 52. These slots or aperturesare spaced equidistant about the broad wall and extend radially outward.The coupling slots 52 are open to the interior of the input cavity 41for the purpose of coupling energy into the input cavity 41.

Thus it is seen that the objects and advantages of the present inventionare obtained in high frequency energy interchange devices of the typewhich rely on an interchange of energy between an electron stream andradio frequency fields. While particular embodiments of the inventionhave been shown, it Will, of course, be understood that the invention isnot limited thereto, since many modifications, both in the circuitarrangements and in the instrumentalities employed, may be made. It iscontemplated by the appended claims to cover any such modifications asfall within the true spirit and scope of the invention.

What I claim is new and desire to secure by Letters Patent of the UnitedStates is:

1. A high frequency energy interchange device including in combination aWaveguide structure adapted to propagate high frequency electromagneticwaves having a transverse electric ON type mode, means for causingpropagation of electromagnetic waves along said waveguide structure insaid transverse electric ON type mode, electron gun means positioned fordirecting a stream of electrons down the length of said waveguide, andperiodic electron stream focusing means for causing the electrons tohave a periodic undulatory motion along the electric flux lines oftransverse electric field of the said electromagnetic waves, saidelectron gun means directing the electron stream down said guide at sucha velocity that the electron stream progresses down said waveguide oneperiod while said electromagnetic wave is propagated substantially oneperiod plus one wave length.

2. In combination in a high frequency energy interchange device awaveguide structure adapted to propagate electromagnetic waves having atransverse electric mode of the ON type, means for causing anelectromagnetic Wave to be supported in said Waveguide structure in saidtransverse electric ON type mode, electron gun means positioned adjacentone end of said Waveguide for directing a stream of electrons down thelength of said guide, and periodic magnetic focusing means positionedadjacent said waveguide whereby electrons in said stream have a periodicundulatory motion along lines of electric flux of said transverseelectric field produced by the said electromagnetic Wave, said electrongun means directing said electron stream down said guide at a velocitysuch that the stream progresses one period while said electromagneticwave propagates substantially one period plus one wavelength.

3. In a high frequency energy interchange device, the combination of arectangular Waveguide structure adapted to support propagation of anelectromagnetic wave of a transverse electric ON type mode, means forcausing an electromagnetic Wave to be supported in said waveguidestructure in said transverse electric ON type mode, an electron gunmeans positioned at one end of said guide for directing a stream ofelectrons down waveguide, a

13 collector means positioned at the opposite end of said guide fordissipating residual energy in said stream and periodic electron streamfocusing means positioned along the length of said waveguide forimparting a substantially periodic undulatory motion to electrons insaid stream which motion is along the lines of electric fiux of thetransverse electric field of said electromagnetic wave, said electrongun means imparting such a velocity to electrons in the stream that theytravel down said waveguide one period while said electromagnetic wave ispropagated substantially a like distance plus one wavelength.

4. A-high frequency energy interchange device for providing interactionbetween a fast electromagnetic wave and a stream of electrons includinga waveguide structure of rectangular cross section adapted to support anelectromagnetic wave having a transverse electric mode of the ON type,means for causing an electromagnetic wave to be supported in saidwaveguide structure in said transverse electric mode, electron gun meansand electron collector means positioned at opposite ends of saidwaveguide for directing electrons down the length of said guide andreceiving electrons, respectively, and a spatially periodic magneticfocusing means positioned near opposite sides of said guide forproducing a periodic magnetic field down the length of said guide whichhas components perpendicular to the direction of flow of electrons insaid guide and in such a direction as to cause said electrons to have aperiodic undulatory motion along introducing an electromagnetic wave insaid waveguide structure for propagation therealong in said mode, meansto direct a hollow stream of electrons coaxially down the length ofsaid-guidebetween inner and outer conductors and spatiallyperiodic-electron stream focusing means down the length 'of said guidefor rotating electrons in said hollow stream in opposite senses alongelectric field lines as they move down the guide length whereby theyhave a periodic undulatory motion, said electron stream directing meansimparting such a velocity to said electron stream that electrons travelthe distance of one period down said waveguide while saidelectromagnetic wave travels a distance which is greater bysubstantially one wavelength.

6. A device of the type defined in claim 5 wherein the said spatiallyperiodic electron stream focusing means is magnetic.

7. In combination in a high frequency energy interchange device of thetype which depends upon an energy interchange between an electron streamand an electromagnetic wave, a waveguide structure of circular crosssection adapted to propagate an electromagnetic wave having a circulartransverse electric mode of the ON type, means for causing propagationof an electromagnetic wave along said waveguide structure in saidcircular transverse electric mode, means to direct a stream of electronsdown the length of said Waveguide, and spatially periodic focusing meanspositioned down the length of said guide to cause electrons in saidstream periodically to undulate back and forth along lines of electricflux of the electromagnetic wave, said means for directing the electronstream imparting such a velocity thereto that electrons in said streamprogress one period while said electromagnetic wave propagatessubstantially one period plus one wavelength.

8. A high frequency energy interchange device as de- 'fined in claim 7whereinsaid spatially periodic focusing means includes a spatiallyperiodic magnetic structure.

9. A high frequency energy interchange device includ- -ing.incombination first and second resonant waveguide structures of like crosssection adapted to be excited in a transverse electric mode of ON type;a drift channel having a cross section similar to said first and secondsections but with dimensions as to preclude propagation of the excitedelectromagnetic waves from said guide, said drift channel beingcoaxially positioned between said first and second guides and defining acontinuous open path therethrough; means for directing a stream ofelectrons down the path through said first waveguide, said drift channeland said second waveguide, respectively; means for excitingelectromagnetic waves in said transverse electric mode in said firstwaveguide for modulating said electron stream, said second waveguidebeing adapted for excitation-of electromagnetic waves therein inresponse to the modulated electron stream; and periodic electron streamfocusing means adjacent said first and second waveguide structures forcausing electrons passing therevthrough to have a periodic undulatorymotion along lines .of electric flux of the transverse electric field ofthe electromagnetic waves, said means for directing the stream ofelectrons imparting a velocity to the stream such that electrons in saidfirst and second waveguide structures progress substantially oneundulatory period while the forward component of the electromagneticwaves in those structures progress one period plus one wavelength.

10. A high frequency energy interchange device of the type defined inclaim 9 wherein the said periodic electron stream focusing meansincludes periodic magnetic means;

11. In a high frequency energy interchange device of the class whichdepends upon interaction between elecltronsiin a stream andelectromagnetic waves the combina- 'a stream of electrons down the paththrough said first waveguide, said drift channel and said secondwaveguide, respectively; means for exciting electromagnetic waves insaid transverse electric mode in said first waveguide for modulatingsaid electron stream, said second waveguide being adapted'for excitationof electromagnetic waves therein in response to the modulated electronstream; and periodic electron stream focusing means adjacent saidfirst-and second waveguide structures for causing electrons passingtherethrough to have a periodic undulatory motion along lines ofelectric flux of the transverse electric field of the electromagneticwaves; said means for directing the stream of electrons impartingvelocity to the stream such that electrons in said first and secondwave-guide structures progress substantially one undulatory period whilethe forward component of the electromagnetic waves in those structuresprogress one period plus one wavelength.

12. A high frequency energy interchange device as defined in claim 11wherein the said periodic electron stream focusing means includesperiodic magnetic means.

13. In combination in a high frequency energy interchange device of thetype which depends upon an energy interchange between an electron streamand an electromagnetic wave, a resonant waveguide structure of circularcross section adapted to be excited by an electromagnetic wave having acircular transverse electric mode of the ON type, means for exciting anelectromagnetic wave in said transverse electric mode for propagation insaid waveguide structure, means to direct a stream of electrons down thelength of said waveguide, and spatially periodic focusing meanspositioned down the length of said guide to cause electrons in saidstream periodically to undulate back and forth along lines of electricflux of the electromagnetic wave, said means for directing the electronstream imparting such a velocity thereto that electrons in said streamprogress one period while the forward component of said electromagneticwave propagates substantially one period plus one wavelength.

14. A high frequency energy interchange device as defined in claim 13wherein said spatially periodic focusing means includes a spatiallyperiodic magnet structure.

15. In combination: a waveguide for propagating electromagnetic wavesalong an axis thereof; an electron gun disposed opposite one end of saidwaveguide for projecting a stream of electrons along said axis; amagnetic structure disposed along the length of said waveguide forproviding a magnetic field having a component thereof directedperpendicularly to said axis, the direction of said magnetic fieldcomponent reversing at intervals along said length for causing saidstream of electrons to undulate in a direction transverse to said axis;and launching means for launching an electromagnetic Wave along saidwaveguide wherein an electric field component of said wave is directedperpendicularly to both said axis and said magnetic field component forproviding interaction between said stream of electrons and said electricfield component in a direction substantially perpendicular to said axis.

16. The combination of claim 15, wherein said launching means isdisposed near one end of said waveguide and further including outputtransmission path means disposed near the other end of said waveguide.

17. The combination of claim 16, further including electron collectormeans disposed opposite the other end of said Waveguide.

18. The combination of claim 17, wherein said waveguide comprises ahollow conductive member and wherein said member is evacuated.

19. The combination of claim 15 wherein said intervals are uniform.

20. The combination of claim 15 wherein said waveguide is rectangular incross-section.

21. The combination of claim 15 wherein said wave guide comprises acoaxial waveguide structure.

22. The combination of claim 15 wherein said waveguide is circular incross section.

23. In combination: an elongated hollow member for supportingelectromagnetic energy therein; means for projecting a stream ofelectrons along the longitudinal axis of said member; a magneticstructure disposed along the length of said member for providing amagnetic field having a component thereof directed perpendicularly tosaid axis, the direction of said magnetic field component reversing atintervals along said length for causing a component of velocity of saidstream of electrons transverse of said axis; and means for causingelectromagnetic energy to be supported in said member wherein anelectric field component of said energy is directed perpendicularly toboth said axis and said magnetic field component for causing amodulation by said electric field component of said transverse componentof velocity of said stream of electrons.

24. In combination: an electron gun for projecting a stream of chargedparticles along an axis; means disposed along said axis for providing asteady magnetic field component oriented perpendicularly to said axis,the direction of said magnetic field component alternating as a functionof distance along said axis for forcing said stream to undulate in adirection transverse to said axis; and means for propagating anelectromagnetic wave along said axis, said wave having an electric fieldcomponent oriented perpendicularly to both said axis and said magneticfield component for providing interaction between said stream and saidelectric field component in a direction substantially perpendicular tosaid axis.

25. The combination of claim 24, wherein the axial velocity of theelectron stream projected by said gun is adjusted for a synchronousrelationship between said stream and said wave, said synchronousrelationship occurring when in the interval required for an electron insaid stream to travel along said axis between two adjacent regions oflike magnetic field component said wave travels along said axis adistance required to provide like electric field components at each ofsaid regions.

References Cited in the file of this patent UNITED STATES PATENTS2,241,976 Blewett et al. May 13, 1941 2,249,494 Ramo July 15, 19412,296,355 Levin Sept. 22, 1942 2,409,991 Strobel Oct. 22, 1946 2,414,121Pierce Jan. 14, 1947 2,591,350 Gorn Apr. 1, 1952 2,650,956 Heising Sept.1, 1953 2,794,936 Huber June 4, 1957 2,808,510 Norton Oct. 1, 19572,860,278 Cook et al Nov. 11, 1958 2,925,520 Cutler et al Feb. 16, 1960FOREIGN PATENTS 995,137 France Aug. 14, 1951 699,173 Great Britain Nov.4, 1953

1. A HIGH FREQUENCY ENERGY INTERCHANGE DEVICE INCLUDING IN COMBINATION OF WAVEGUIDE STRUCTURE ADAPTED TO PROPOGATE HIGH FREQUENCY ELECTROMAGNETIC WAVES HAVING A TRANSVERSE ELECTRIC ON TYPE MODE, MEANS FOR CAUSING PROPAGATION OF ELECTROMAGNETIC WAVES ALONG SAID WAVEGUIDE STRUCTURE IN SAID TRANSVERSE ELECTRIC ON TYPE MODE, ELECTRON GUN MEANS POSITIONED FOR DIRECTING A STREAM OF ELECTRONS DOWN THE LENGTH OF SAID WAVEGUIDE, AND PERIODIC ELECTRON STREAM FOCUSING MEANS FOR CAUSING THE ELECTRONS TO HAVE A PERIODIC UNDULATORY MOTION ALONG THE ELECTRIC FLUX LINES OF TRANVERSE ELECTRIC FIELD OF THE SAID ELECTROMAGNETIC WAVES, SAID ELECTRON GUN MEANS DIRECTING THE ELECTRON STREAM DOWN SAID GUIDE AT SUCH A VELOCITY THAT THE ELECTRON STREAM PROGRESS DOWN SAIDE WAVEGUIDE ONE PERIOD WHILE SAID ELECTROMAGNETIC WAVE IS PROPAGATED SUBSTANTIALLY ONE PERIOD PLUS ONE WAVE LENGTH. 